Fischer-Tropsch carbon monoxide hydrogenation is a catalysed process in which a carbon monoxide and hydrogen mixture, typically referred to a “synthesis gas” or “syngas”, is converted into liquid hydrocarbons, predominantly linear hydrocarbons (olefins and paraffins) of different chain length. Small amounts of oxygenates, mainly alcohols and aldehydes, can also be formed. The products can be used to produce transportation fuels such as diesel and petrol. Alternately, or in addition, valuable chemicals such as olefins and oxygenates can be extracted from the product.
The synthesis gas can include carbon dioxide as well as carbon monoxide or water as well as hydrogen, and may also include combinations of all four of these reactants.
There is a considerable difference in the value of paraffins and olefins with mixtures predominating in paraffins being of relatively low value and primarily usable for the production of transportation fuels and waxes, whilst olefins themselves, are of substantially greater value and are usable for numerous other purposes.
In recent times, cobalt based catalysts have become an attractive alternative to iron based catalysts as those most widely used in Fischer-Tropsch reactions. Studies have reported that nitrogen containing compounds act as a poison or inhibitor to these catalysts. In an attempt to overcome this, published international patent application number WO 2005/071044 discloses a process for pre-treating a catalyst such that it loses no more than 50% of its activity in the presence of trace amounts of ammonia (parts per billion levels) in a feed to a Fischer-Tropsch reaction.
WO 2001/049809 and WO 2001/049808 describe a process in which ammonia is co-fed to the reduction gas during catalyst pre-treatment. An increase of C5+ selectivity and chain growth probability as well as a decrease in methane selectivity was reported.
A further problem often encountered with Fischer-Tropsch type reactions is that it is very difficult to selectively control the production of valuable products, especially olefins.